Fluid-operated electrical switch



Jan. 27, 1970 s o. AYREs ETAL FLUID-OPERATED ELECTRICAL SWITCH 3Sheets-Sheet 1 Fig.2.

Filed Feb. 28, 1967 Fig. 1.

1970 s. D. AYRES ETAL FLUID-OPERATED ELECTRICAL SWITCH a Sheets-Sheet 2Filed Feb. 28, 1967 1970 s. D. AYRES' ETAL FLUID-OPERATED ELECTRICALSWITCH 3 Sheets-Sheet 3 Filed Feb. 28, 1967 United States Patent3,492,612 FLUID-OPERATED ELECTRICAL SWITCH Sidney Dennis Ayres, JohnChristopher Hammond Davis, and Kenneth Hutton, Taplow, and David JohnCraft, Chandlers Ford, England, assignors to ,British TelecommunicationsResearch Limited, Taplow, England, a British company Filed Feb. 28,1967, Ser. No. 619,263 Claims priority, application Great Britain, Mar.3, 1966,

9,450/66 Int. Cl. H01h 29/28 US. Cl. 335-112 12 Claims ABSTRACT OF THEDISCLOSURE A mercury contact switch consists of a tube having two fixedcontacts protruding through the walls near one end. The bead of mercury,disposed in the tube, is movable by fluid, preferably air pressurebetween one end of the tube and the other. When the bead of mercury isat one end of the tube it makes connection between the two fixedcontacts. Preferably the ends of the tube are closed by porous plugs.

The present invention relates to electric switching devices and isparticularly concerned With devices suitable for use in light currentengineering, particularly telecommunications.

In automatic telephone systems, which may be consldered as a typicalexample, connections are generally made by the selective closure of someof a group of contacts, and since these contacts will often be carryingspeech currents, it is important that the contact should be a good oneand made reliably even after a large number of operations which mayamount to millions. An obvious possibility is to make use of a highlyconductive liquid and for this purpose mercury is very suitable. Agenerally similar use is already Well known in connection with contactthermometers where contacts which need to be closed at a specifictemperature are fused into the bore of a mercury thermometer at theappropriate place. For such use, however, speed of operation is notlikely to present any problems, and clearly the mercury is not likely tomove very fast. Where the operation of a switch is effected by tilting atube containing mercury as is also well known, the arrangement tends tobe somewhat cumbersome for small currents and is quite slow in thecontext of the telecommunications art. The chief object of the presentinvention is to provide an improved mercury switch which occupies only asmall space, requires little power for its operation, gives reliablecontacts, and can be operated at high speed.

According to the invention, an electrical switching device comprises aclosed channel of small cross-section containing a bead of anelectrically conductive liquid and having walls of an electricallynon-conductive material through which first and second contacts protrudeinto said channel, said contacts being so disposed that the bead ofelectrically conductive liquid makes contact therebetween when in afirst position in the channel and does not make contact therebetweenwhen in a second position in the channel, in which said bead is arrangedto be moved between said first and second positions by pressure exertedthereon by a non-conductive fluid, said pressure being arranged to begenerated by means external to said fluid.

Preferably the movement is controlled by means of a flexible diaphragmwhich may be spaced away from the mercury and may itself be operated byfluid pressure on the opposite side or may be moved mechanically orelectromagnetically or piezoelectrically. Such a diaphragm can readilybe arranged to control a plurality of beads of 3,492,612 Patented Jan.27, 1970 mercury simultaneously, thereby giving the same effect as amulti-contact electromagnetic relay. This facility is of importance inautomatic telephone systems where switches in the speech circuits areusually required to operate at least three contacts simultaneously.

In practice, it is found desirable to limit the movement of the mercuryby a buffer of sintered or other suitable material which is imperviousto mercury but permits fluid pressure to be readily transmittedtherethrough. Such buffers are provided at each end of the travel of themercury and generally the design will incorporate a fluid cushion at theend remote from the operating diaphragm.

For most control purposes, it will not be possible to isolate the switchfrom ambient temperature and pressure variations, which will affect theinternal pressure and hence the operation of the switch. To deal withthis, it may be desirable to provide a bleed channel between theopposite sides of the mercury bead to enable pressure equalisa tion totake place at a slow rate after a switching operation has taken place.If the tube in which the bead is located is of circular cross-section,it will generally be advisable to provide a separate bleed channel whichmay, however, be common to a number of separate switches which operatesimultaneously. If, however, the section of the channel is rectangularor D-shaped as might be produced by moulding a channel in a block ofmaterial and closing it with a cover plate, the mercury will not readilymove into the corners of the section owing to surface tension effects,and this may provide a suflicient leak to obviate the necessity for aseparate bleed channel.

The contacts themselves, which are bridged by the mercury, may be of anysuitable contact metal, but preferably one which is wetted by themercury, for instance silver.

According to the method of switching required, it may be desirable thatthe movement of the mercury bead should accurately follow the movementof the diaphragm, but for some purposes it might be desirable to producea bi-stable characteristic, and this may readily be attained by usingwhat may be termed a waisted capillary, that is to say one in which thebore is narrower in the middle than at the ends. With this arrangement,once the mercury has been pushed past the waist, it will tend to remainin that position when the fluid pressure difference across it whichcaused its movement decays or is removed. Thus, as suggested above, theleak or bleed will cause the pressure difference to disappear even ifthe operating input pressure is maintained.

The switch according to the invention leads itself to grouping on across-point basis where the switches are arranged in rows and columns toform a matrix, and a switch is operated at the point common to a chosenrow and column. This concept may be extended to the design of a switcharranged to simulate the operation of a standard cross-bar switch inwhich contacts are closed at a particular cross-point in response to theselection of appropriate rows and columns and the connection is retainedafter one of the original operating forces is removed, conditions thenbeing returned to normal when the other operating force is also removed.

The invention will be better understood from the following descriptionof several methods of carrying it into effect, which should be taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a partially broken away plan of one embodiment arranged on across-point basis and using commoning conductors connecting therespective rows and columns;

FIG. 2 is an exploded sectional view taken on the line IIII in FIGURE 1;

FIG. 3 shows a possible electro-magnetic method of operating theactuating diaphragm;

FIG. 4 shows a modified section of the tube in which the mercury beadoperates;

FIG. 5 indicates how the use of a tube of square section automaticallyproduces narrow bleed channels owing to the surface tension of themercury;

FIG. 6 is a plan of a modified form of switch according to the inventionwith a top cover plate removed;

FIG. 7 is a sectional view taken on the line VII-VII in FIGURE 6 butshowing the cover plate;

FIG. 8 is a diagrammatic plan view of a modified construction in whichthe mercury bead is normally in two parts which are united whenoperating pressure is applied to the diaphragm;

FIG. 9 is a sectional view taekn along the line IXIX in FIG. 8; and

FIGS. 10-14 show diagrammatically a construction from which regularcross-bar operation may be obtained, these figures indicating differentstages in the operation.

Referring first to FIGURES 1 and 2, it is assumed that the switch isconstructed on a row and column basis and that at each cross-point fourseparate contacts are obtainable as may be required in an automatictelephone system. The conductors for the rows are indicated at 1 and theconductors for the columns at 2, and these conductors are located inseparate parallel, e.g. horizontal, planes spaced slightly apart. Theyare assumed to be embedded in glass or plastics material, preferably ofthe transparent type, by suitable moulding operations. Thus the columnconductors may be disposed on a base member 3 of plastics material andcovered with a thin sheet of material. The row conductors are then putin position and are covered with a further sheet of material, andfinally the whole is fused together, by the application of suitable heatand pressure. At any one cross-point, four holes 4 are drilled throughthe composite sheet so as to pass through corresponding row and columnconductors, the holes thus forming a diagonal of the 4 x 4 matrix formedby the two sets of parallel conductors. These holes are preferablyarranged to be convergent/divergent as shown in FIGURE 2; in otherwords, to have a waisted portion in the middle. Each of the holescontains a bead of mercury 5 which fills approximately half of the hole,that is to say up to the waisted portion as indicated in FIGURE 2.

The holes are covered at each end by the stops 6 and 7,

On the lower side, as sohwn in FIGURE 2, the various cross-points areenclosed by a similar plate which however, provides a chamber 9individual to each cross-point.

Each chamber 9 includes a diaphragm10 opposite which is a hole throughwhich operating fluid is supplied.

As mentioned above, in order to cater for changes in ambient temperatureand pressure, it is desirable to provide a bleed, and this is effectedin the construction shown by the device 11 which is arranged to offerhigh resistance to fluid flow. It will be understood that the bleedshould provide equalisation of pressure in a time -which is smallcompared with the rate of such pressure changes but is large comparedwith the operating time of the switch. Since it is visualised that theswitch would operate in a time of the order of one millisecond, whileambient changes, even if produced by the heating effect of associatedelectrical equipment, would almost certainly require a matter of minutesto produce their effect, it is contemplated that the time forequalisation of pressure to tak p a by the b e d m ght a abes wa d- Oneform of construction which might be adopted is shown in FIGURE 5, inwhich it is assumed that the bleed compresses a tube of square sectionwhich is also filled with mercury and extends between the two stops 6and 7. Owing to the surface tension effect, the mercury will not extendfully into the corners of the section, and this permits very slowmovement of operating fluid so as to produce the necessary equalisationeffect. Of course, the cross-section of the channel need not be square.It can have any angular shape provided that one of the internal anglesthereof is less than the complement of the angle of contact of theelectrically conductive liquid.

It will be understood that when pressure is applied to deflect thediaphragm 10, a pressure pulse will be effective through the stop 7 tocause the mercury beads to move to the other end of their channels,thereby completing contact between the row and column conductors. Owingto the waisted section of the channels, there will be a tendency for themercury beads to stay in the operative position owing to surface tensioneffects and in any event reliable contact will be maintained even if thedevice is subject to some degree of vibration which otherwise mightcause the mercury to restore. The opening of the switch can then beeffected by moving the diaphragm in the opposite direction so as toexert a suction effect.

FIGURE 3 shows how, if the diahpragm is provided with an armature or isin fact made of flexible magnetic material, the necessary switchingeffect may be produced by a coil, so that if the control of the switchis electric, no separate transducing operation is required.

FIGURE 4 shows another form of tube which may be used in which thesection in which the mercury makes contact is parallel-sided, while thelower section which accommodates the mercury in the open position of theswitch is convergent. Further, at least part of the bi-stable action canbe produced by mercury-Wetted inserts at each end of the tube.

Considering now the second embodiment shown in FIGURES 6 and 7, this isof the type in which the tubes in which the mercury operates are not ina vertical plane as is the case with the FIGURES 1 and 2 constructionbut in a horizontal plane, and they are formed by moulding channels inthe surface of a base member and closing these channels by means of acover plate. As in the previous arrangement, the base member is assumedto be of glass or plastics material and is of the circular shape shownat 14. It is provided with moulded channels 15 of double-conical waistedsection, which are terminated at one end by a stop 16 and at the otherend by a similar stop 17, these being of sufficient size to cover theends of all the four channels shown. Each of these channels contains abead of mercury 18 which approximately half fills it. The channels areclosed by a cover plate 19, which carries pairs of contacts 20, whichprojects sufficiently below the plate to dip into the mercury when it ismoved to the other end of the channels, and above the plate provideconnections for external wiring. The lower portion of the base 14accommodates the operating diaphragm 21 which defines a chamber 22communicating by means of a suitable orifice 22a with a chamber 24formed behind the stop 16. A similar chamber 23 is formed behind thestop 17 and this communicates via a channel 25a with the annular chamber25, which may if desired be made common to a plurality of switchingpoints. This chamber is closed by a lower cover plate 26 so as to form achamber 27 beneath the diaphragm to which the operating fluid atpressure P is applied by way of the pipe 28. As before a number ofindividual cross-points may be combined to form a matrix and commoningbetween the different conductors may be effected by means of ribboncable or the like connected to the upper ends of the contacts 20. Thesecables might, for instance, extend diagonally in two directions at rightgl s. in l cn. o F GU E 6. t wi l be pp i ted that with this arrangementthe section of the channels is not circular and thus the mercury doesnot enter completely into the corners, thus allowing minute channelsthrough which pressure equalisation can take place. Accordingly, aseparate bleed will not be necessary as the constructionin effectprovides a built-in bleed.

Referring now to FIGURES 8 and 9 these show an alternative arrangementin which two beads of mercury 41 and 42 are normally located at oppositeends of a dumb-bell-shaped groove 43 with a hump 44 in the middleportion, each of the end portions 45 and 46 having a contact (not shown)extending into the mercury. The groove is covered by a domed circulardiaphragm 47 so as to prevent escape of the mercury, and operation iseffected by applying mechanical pressure to the centre of the diaphragm47. The pressure generated thereby in the mercury acts against thesurface tension of the two mercury beads 41 and 42 which are forcedalong the groove 43 so that they join and thus complete a circuitbetween the two contacts. This condition will be maintained until thepressure on the diaphragm 47 is removed, when the mercury will againseparate into two beads 41 and 42. Obviously, with this constructionfrom the diaphragm 47 must be made of material which, does not reactundesirably with the mercury. Moreover, in the simple form shown, theswitch is unlikely to be satisfactory if it is subjected to seriousvibration or movement involving appreciable acceleration.

Consider now the embodiment of the invention shown in FIGURES 10-14,which is suitable for providing a method of operation similar in mostrespects to that of a cross-bar Switch. In these figures, which areintended to be only diagrammatic, the mercury 29 is contained within avertical tube 30, and is prevented from escaping by the stop 31 at thelower end. At the other end, the tube is separated by a stop 32 from acavity 33 of large size compared with the displacement of the mercury29. The contacts 34 and 35 to be bridged are located in the upperportion of the tube, just below the stop 32. The tube is provided with aside tube 36 and this is closed by a stop 37 which prevents the mercurymoving into it.

In the normal open position, the mercury 29 is maintained in the lowerpart of the tube by gravity or surface tension effects if this part isof larger section or of wetted metal, and the contacts are not made andthere is no excess pressure in the cavity or applied to either of thecontrol inlets. When the switch is to be set, pressure P is applied tothe tube below the stop 31 corresponding to the operation of a selectmagnet in a normal cross-bar switch and this has the effect of movingthe mercury bead to the other end of the tube, thereby closing thecontacts as shown in FIGURE 11. Pressure equal to 2P is now applied tothe side tube 36 corresponding to the operation of a bridge magnet, andthis has the the effect of dividing the mercury into two heads, one 39of which fills the upper portion of the tube and maintains the contactsclosed, while the other 40 rests on the stop 31 as shown in FIGURE 12.The original pressure applied to the lower end of the tube may now beremoved and the contacts will remain closed until pressure is removedfrom the side tube corresponding to the release of the bridge magnet ofa normal cross 'bar switch. When release is to take place and this:pressure also is removed, the upper bead 39 which is bridging thecontacts now falls by gravity and joins up with the lower beads 40 asindicated by the arrow A in FIGURE 13. In order to ensure that thisshall take place satisfactorily, a passage 38 is provided which permitsfluid to escape from between the two beads so that they are able tocoalesce.

A further facility offered by the switch is shown in FIGURE 14 whereby,if pressure 2P is applied to the side tube before pressure P is appliedat the lower end, no movement of the mercury will take place when bothpressures are effective, and the switch will remain in the 6 OFFposition. Obviously, in these circumstances a pressure of 2P will alsobe developed in the cavity 33.

It will be understood that though it is suggested that the buffers ofstops would be of sintered or ceramic material, other material of aporous nature may be used and the plugs produced may be fused inposition or retained mechinically, for instance by spring loading.Similarly, the operating diaphragm may be made of any suitable materialwhich will provide the necessary flexibility, will not react withmercury, vapour or liquid mercury, and can be readily sealed into itssupport. While the operation of the diaphragm can generally be mostadvantageously effected by fluid pressure, it can, however, be effectedby direct mechanical action, electro-mechanically or piezoelectriqially.Furthermore, in embodiments where the diaphragm is not required toprovide resilience, it may in fact take the form of a free mercury beadsealing a surrounding tube. Such an arrangement may be extended toincorporate a mercury-floated inductive needle which can be moved by amagnetic field to operate the switch.

The invention accordingly provides a novel switch arrangement which, byvirtue of its compact construction, rapidity of operation, andpossibility of fluid control, offers important advantages in the fieldof telecommunications.

We claim:

1. An electrical switching unit comprising first and second chambers, aplurality of closed channels interconnecting said first and secondchambers, each of said channels having walls of an electricallynonconductive material, respective pairs of electric contacts projectinginto each of said channels, a respective bead of an electricallyconductive liquid disposed in each channel, means communicating with oneof said two chambers for applying a fluid pressure differential betweensaid two chambers to move each of said beads of electrically conductiveliquid between a first position in proximity to said first chamber inwhich it establishes connection between the corresponding pair ofelectrical contacts and a second position in proximity to said secondchamber in which it does not make such connection.

2. A unit as claimed in claim 1, including a pair of buffers for eachchannel, said buffers being made of a material which is impervious tothe electrically conductive liquid but which permits fluid pressure tobe transmitted therethrough, one of said pair of buffers being adaptedto close the end of its respective channel adjacent said first chamberand the other of said pair of buffers being adapted to close the end ofits respective channel adjacent said second chamber.

3. A unit as claimed in claim 1, in which one of said chambers containsa diaphragm adapted to divide said one chamber into a first portion anda second portion with all said channels communicating with said firstportion, whereby deflection of said diaphragm is effective to generatesaid pressure differential.

4. A unit as claimed in claim 3, including means for applying fluidpressure to a second of said two portions.

5. A unit as claimed in claim 3, including an electromagnet disposedconfronting said diaphragm and means associated with said diaphragmwhereby said diaphragm is movable under the influence of the magneticfield generated when said electromagnet is energised.

6. A unit as claimed in claim 5 in which a member of magnetic materialis mounted on said diaphragm, said diaphragm being made of flexiblenon-magnetic material.

7. A unit as claimed in claim 5, in which said diaphragm is made of aflexible magnetic material.

8. A unit as claimed in claim 1, in which a bleed duct is arranged toconnect said first chamber to said second chamber.

9. An electrical switching arrangement comprising a plurality ofswitching units as claimed in claim 1, said units being arranged in amatrix comprising rows and columns,

a respective first group of electric commoning connections for each row,each commoning connection of said first group connecting a first contactof the contact pair in a respective channel of a device in the row tofirst contacts of the contact pairs in corresponding channels in theother devices in the row, and a respective second group of electricalcommoning connections for each column, each commoning connection of saidsecond group connecting a second contact of the contact pair in arespective channel of a device in the column to second contacts ofcontact pairs in corresponding channels in the other devices in thecolumn.

10. An electrical switching device comprising a body portion of anelectrically non-conductive material and having a dumb-bell shapedgroove in the upper surface thereof, the central part of said groovebeing humped, a pair of electrical contacts each disposed in arespective end portion of said dumb-:bell shaped groove, two beads of anelectrically conductive liquid disposed in respective end portions ofsaid groove and a domed diaphragm secured to said body portion andcovering said groove whereby, on depression of said diaphragm, the twobeads of electrically conductive liquid are forced into the humpedcentral part of the groove and join together to establish connectionbetween the two electrical contacts.

11. An electrical switching device comprising a closed channel, twobuffers made of a material which is impervious to liquid but whichpermits gas pressures to be transmitted therethrough disposed inrespective ends of the channel a closed cavity connected to one end ofthe channel, a side duct connected to the channel between the twobuffers, a further buffer of a similar material to said two buffersclosing the side duct at the point of connection with the channel, apair of electrical contacts projecting into the channel between the sideduct and the butter adjacent to the close cavity, and a bead of anelectrically conductive liquid disposed in said channel and having alength greater than the distance between the side duct and the bufferadjacent to the cavity but less than the distance between the side ductand the buffer remote from the cavity, where by application of gas at afirst pressure to the end of the channel remote from the cavity causesthe bead to take up a position in contact with the buffer adjacent tothe cavity and subsequent application of gas at a second pressuregreater than the first said side duct causes said bead to divide intotwo parts at the point of connection of said side duct, a first partremaining in contact with the buffer adjacent to the cavity to maintainelectrical connection between the contacts and a second part returningto the end of the channel remote from the cavity.

12. A device as claimed in claim 11, in which a by-pass duct extendsfrom the channel at a position adjacent to the end of the bead remotefrom the cavity when said bead is in the position taken up afterapplication of said first pressure and prior to application of saidsecond pressure to the side duct, said by-pass duct being separated fromthe end of the side duct remote from the channel by a buffer similar tothose at the ends of the channel.

References Cited UNITED STATES PATENTS 2,611,057 9/1952 Slonneger 2008l.6 3,271,543 9/1966 Schonfeld 200-81.6 3,328,543 7/1967 Johnston20081.6 2,744,980 5/1956 Bellamy 33549 BERNARD A. GILHEANY, PrimaryExaminer H. BROOME, Assistant Examiner US. Cl. X.R. 200'8l.6

